Air bags have hitherto been produced mainly from coated fabrics, but it has been found that these fabrics have considerable disadvantages for the production of air bags. Apart from the higher production costs required for coating the fabrics, the folded volume of the coated fabrics is at least 10% greater than that of uncoated fabrics. The space requirement for accommodating the air bag, for example in the steering wheel, is therefore higher when the fabrics are coated than when they are uncoated. Moreover, air bags of coated fabrics result in a greater imbalance in the steering wheel than those produced from uncoated fabrics. Another special disadvantage arises from the necessity of applying talc to the coating to prevent the different layers of coated fabric from sticking together when the air bag is folded up. When the air bag is operated, the talc from the air bag causes considerable discomfort to the passengers.
There is therefore a need to develop uncoated fabrics which could be used for the production of air bags without the disadvantages of the coating described above.
Such a fabric is described in EP-A 314,867, in which a yarn having a titre of 470 dtex is proposed for a fabric with an asymmetric fabric construction The low permeability to air required of these fabrics is obtained by thermofixing in combination with calendering. One of the major disadvantages of the fabrics with asymmetric construction described in said specification is that the same strengths cannot be obtained in both thread directions. Moreover, the titre of 470 dtex is not advantageous for ease of folding of the fabric.
An asymmetric fabric having a titre in the range of from 110 to 900 dtex is described in U.S. Pat. No. 3,842,583, but the yarn used in that fabric has insufficient strength and too high elongation. With yarns of this type it is not possible to fulfill the specifications nowadays demanded by European motor car manufacturers. These asymmetric fabrics also have the further disadvantage of difference in strength between warp and weft.
Fabrics for air bags produced from yarns with titres of 235 dtex (210den) or 470 dtex (420 den) are described in JP-A 01-104,848 (see Examples).
Whereas the titre of 235 dtex does not provide the necessary strength, the titre of 470 dtex has the disadvantage already mentioned above of producing a fabric with too great a folding volume.
The problem therefore arose of developing an uncoated air bag fabric which would be more suitable and inexpensive to produce.